U.S. patent number 4,801,185 [Application Number 07/032,783] was granted by the patent office on 1989-01-31 for directional coupler.
This patent grant is currently assigned to STC PLC. Invention is credited to Terry Bricheno.
United States Patent |
4,801,185 |
Bricheno |
January 31, 1989 |
Directional coupler
Abstract
A fused tapered optical fibre directional coupler for use in a
polarisation-preserving fibre system has a short stub length of
non-polarisation-preserving fibre spliced into each fibre from
which the coupler is made. The reduced diameter and tapered
portions of the coupler are confined to the stub lengths since it
is found that low loss coupling is not achieved if these portions
are fabricated in fibre with a polarisation-preserving
structure.
Inventors: |
Bricheno; Terry (Exxex,
GB3) |
Assignee: |
STC PLC (London,
GB2)
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Family
ID: |
10598374 |
Appl.
No.: |
07/032,783 |
Filed: |
March 31, 1987 |
Foreign Application Priority Data
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May 23, 1986 [GB] |
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8612660 |
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Current U.S.
Class: |
385/42; 385/43;
385/51; 385/96 |
Current CPC
Class: |
G02B
6/2835 (20130101); G02B 6/2843 (20130101) |
Current International
Class: |
G02B
6/28 (20060101); G02B 006/02 (); G02B 006/26 () |
Field of
Search: |
;350/96.15,96.16,96.29 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0148569A |
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Jul 1985 |
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EP |
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0191235A |
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Aug 1986 |
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EP |
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0174014A |
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Dec 1986 |
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EP |
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2150703 |
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Jul 1985 |
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GB |
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Other References
B S. Kawasaki et al, "Low-Loss Access Coupler for Multimode Optical
Fiber Distribution Networks", Applied Optics, vol. 16, No. 7, 1977.
.
B. S. Kawasaki et al, "Biconical-Taper Single-Mode Fiber Coupler",
Optics Letters, vol. 6, No. 7, Jul. 1981. .
Electronics Letters, vol. 21, No. 6, Mar. 1985, pp. 249-251, M. S.
Yataki et al., "All-Fibre Polarizing Beamsplitter". .
IEE Proceedings vol. 132, part J, No. 5, Oct. 1985, pp. 297-302, G
Georgiou et al., "Low-Loss Single-Mode Optical Couplers". .
Electronics Letters vol. 19, No. 16, 4th Aug. 1983, p. 601, R. B.
Dyott et al., "Polarisation-Holding Directional Couplers Made From
Elliptically Cored Fibre Having a D Section"..
|
Primary Examiner: Sikes; William L.
Assistant Examiner: Ullah; Akm E.
Attorney, Agent or Firm: Lee & Smith
Claims
I claim:
1. A fused tapered optical fibre directional coupler, which coupler
is constructed from a plurality of lengths of single mode fibre
that have been arranged in side by side assembly and drawn down and
fused together to produce a reduced diameter portion of the
assembly within which reduced diameter portion the fibres are
mutually coupled, wherein each one of said plurality of lengths of
single mode fibre is formed of two axes-aligned portions of
polarisation-preserving fibre spliced to opposite ends of an
associated stub of non-polarisation-preserving fibre, and wherein
said stubs accommodate the reduced diameter portion of the assembly
within which the fibres are mutually coupled.
2. A directional coupler as claimed in claim 1 which coupler is
constructed from two lengths of single mode fibre each
incorporating its stub of non-polarisation-preserving fibre spliced
between two axes-aligned portions of polarisation-preserving fibre.
Description
BACKGROUND OF THE INVENTION
So-called single mode optical fibre that is circularly symmetric is
in fact capable of supporting two degenerate orthogonally polarised
modes. Bends, twists and other irregularities are liable to
introduce coupling between these modes, with the result that the
polarisation state with which light is launched into one end of
such fibre is soon changed as the light propagates along the fibre.
In many coherent light optical fibre communications systems and
sensor systems the resulting indeterminate change of state of
polarisation (SoP) is unacceptable in which case use is made of
fibre in which the degeneracy of the two zero-order modes is
removed. The two modes are arranged to have sufficiently different
propagation constants for the bends, twists and other
irregularities encountered by the fibre to introduce substantially
no coupling of the modes, and hence the separate identity of the
two modes is preserved as light propagates down such fibre. Mode
degeneracy is removed by making the fibre deliberately
birefringent.
For the purposes of this specification single mode optical fibre
which is designed to be sufficiently birefringent to inhibit
coupling between its zero-order modes will be referred to as
polarisation-preserving fibre, also known as high-birefringence
fibre. Single mode fibre which has not been designed to be
birefringent, and which therefore either exhibits no birefringence,
or for which residual birefringence is too small to be effective in
the prevention of mode coupling, will be referred to as
non-polarisation-preserving single mode fibre.
Many of the systems that require the use of polarisation
maintaining fibre in order to avoid the generation of indeterminate
changes of SoP also require the use of branching and/or tapping
elements in their optical fibre networks. Such elements are
conveniently implemented in optical fibre as optical fibre
directional couplers. In ordinary, non-polarisation maintaining
fibre low insertion loss examples of such elements are
advantageously provided by optical fibre directional couplers
created by the method described in United Kingdom Patent
Specification No. 2150703A (U.S. Pat. No. 4,591,372). Satisfactory
results have however not been obtained when attempting to make such
directional couplers of high stress-induced birefringence
polarisation-preserving fibre.
SUMMARY OF THE INVENTION
The present invention is concerned with the provision of adequately
low loss single mode fibre directional couplers for use in
polarisation-preserving fibre networks.
According to the present invention there is provided a fused
tapered optical fibre directional coupler, which coupler is
constructed from a plurality of lengths of single mode fibre that
have been arranged in side by side assembly and drawn down and
fused together to produce a reduced diameter portion of the
assembly within which reduced diameter portion the fibres are
mutually coupled, wherein each one of said plurality of lengths of
single mode fibre is formed of two axes-aligned portions of
polarisation-preserving fibre spliced to opposite ends of an
associated stub of non-polarisation-preserving fibre, and wherein
said stubs accommodate the reduced diameter portion of the assembly
within which the fibres are mutually coupled.
The problem associated with trying to fabricate directional
couplers directly in high stress-induced birefringence fibre
results from the unfavourable refractive index profile that is
associated with the provision of the required stress pattern. The
usual technique for inducing stress-birefringence into a fibre
structure involves the provision of heavily doped regions of the
cladding in regions adjacent the optical core. These heavily doped
regions having a significantly different thermal expansion from the
rest of the fibre, and the stress results from the effects of
differential thermal expansion when the fibre is cooled to room
temperature from the high temperature at which it was drawn from
fibre preform. Associated with the heavy doping to induce the
intended differential thermal expansion effect produced by the
differential doping, is an unintended differential refractive index
effect. When such a fibre is tapered down in size in an attempt to
make a directional coupler, too much light is coupled into unwanted
modes associated with the stress-inducing structure instead of into
the zero-order core modes of the other fibre or fibres of the
directional coupler. The consequence is that the resulting coupler
does not function properly, but exhibits an unacceptably high
insertion loss.
According to the teachings of the present invention this problem is
circumvented by substituting non-polarisation-preserving fibre for
the polarisation preserving fibre over the region where mutual
coupling of the fibres is arranged to occur. Although the inserted
non-polarisation-preserving fibre contains no structure to ensure
that the SOP of light launched into one end propagates to the other
end with substantially no change of SOP, nevertheless such a
preservation of SOP is found in practice to occur. This is because
the distance is relatively short, and over this distance the fibre
is restrained against the sorts of bends, twists and irregularities
that would otherwise be liable to induce significant changes of
SOP.
BRIEF DESCRIPTION OF THE DRAWINGS
There follows a description of the manufacture of a directional
coupler embodying the present invention in a preferred form. The
description refers to the accompanying drawings in which :
FIGS. 1 to 3 depict successive stages in the manufacture of the
directional coupler.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
A length of polarisation-preserving single mode optical fibre
complete with plastics protective coating 1 is cleaved into two
pieces 2, 3 to enable a stub length 4 of
non-polarisation-preserving single mode fibre to be inserted in
between the cleaved ends. Conveniently this insertion is effected
by first forming a fusion splice between one of the pieces of
polarisation-preserving fibre and a piece of
non-polarisation-preserving fibre (not shown), and then re-cleaving
the fibre a short distance along the non-polarisation-preserving
fibre from the splice, before connecting the second piece of
polarisation-preserving fibre by means of a second fusion splice.
In making the second fusion splice, care needs to be taken that the
axes of the two pieces of polarisation-preserving fibre are aligned
with each other.
A second length of identical polarisationpreserving fibre complete
with plastics protective coating 5 is similarly divided into two
pieces 6, 7 for the insertion of its stub length 8 of
non-polarisation-preserving fibre.
The two lengths of fibre are then assembled for making a
directional coupler. For this purpose the fibres need to be in
glass-to-glass contact with each other along the region that is to
be drawn down to provide the mutual optical coupling. This region
of contact is conveniently provided by stranding the fibres, as
illustrated in FIG. 1, while taking care that the stranding
introduces substantially no twist of either fibre about its own
axis. Preferably the stranded fibres are clamped between
independently driven carriages 9 and 10 for the making of the fused
directional coupler by the progressive stretching method
substantially as described in United Kingdom Patent Specification
No. 2150703A now (U.S. Pat. No. 4,591,372).
According to this method the fibres are heated in a flame 11 of a
burner 12 to produce local softening of the glass while the fibres
are being moved in a longitudinal direction through the flame. To
this end the leading carriage, carriage 9 for a rightward traverse,
carriage 10 for a leftward traverse, is traversed at a slightly
faster rate than the trailing carriage. This stretching process is
repeated a number of times until the requisite amount of coupling
has been produced. Typically, though not essentially, successive
traverses are performed in opposite directions. The extent of the
traverses are limited so that the stretching is confined
exclusively to the stub lengths 4 and 8 of
non-polarisation-preserving fibre. In a typical example these stub
lengths are initially about 5 to 6 mm long, and the central 1mm is
stretched to about 5mm.
When the desired coupling has been reached the fibres are mounted
upon a substrate. Conveniently this may take the form of a rod 13
(FIG. 3), made for instance of fused silica, to which the fibres
are secured by the application of quantities of adhesive resin
applied at locations 14 and 15 where the fibres emerge from their
plastics protective coatings. In this way the portion of the fibres
between the two resin attachment points 14 and 15 is held in a
substantially strain-free fashion. This ensures that polarisation
state is well retained even in regions where the light is
propagating in non-polarisation-preserving fibre. This in turn
means that the stub lengths 4 and 8 of fibre can, if required, be
made significantly longer than is necessary solely to accommodate
the stretched region.
For further mechanical protection the assembly may be placed in a
long slender box (not shown) where its ends are anchored before the
interior of the box is filled with a soft potting compount such as
a silicone rubber. Alternatively, the shape of the substrate to
which the fibres are secured may be such that it can be filled with
the potting compound after attachment of the fibres to an interior
surface thereof.
Although the foregoing description has related specifically to the
construction of a 4-port directional coupler constructed from two
fibres, it is to be clearly understood that the invention is
applicable also to directional couplers with more than 4 ports,
particularly 6-port couplers constructed from three fibres.
* * * * *